Ultrasonic surgical instruments for minimally invasive procedures such as liposuction have been described in the prior art. Typically such instruments comprise an elongate tip or horn as these are commonly referred to, a sheath covering the horn, an irrigation source, a vacuum source, a transducer and a power supply. The ultrasonic surgical instruments generally employ piezoelectric or magnetostrictive transducers that transform high frequency electrical energy, i.e. greater than 20,000 Hz, into mechanical vibrations or impulses. The resultant longitudinal waves created are above the audible range and are thus "ultrasonic". The horn transmits these ultrasonic vibrations which when contacted with biological tissue such as fat, break down the cellular structure and allow removal via suction typically through a hollow tube formed inside the horn. In addition, the sleeve provides an annular space to supply irrigation fluid to the operative site and for cooling of the tip, the operating site and the sheath.
As explained in U.S. Pat. No. 5,123,903 (Quaid et al.), there is also a need to provide a sleeve or sheath around the tip to protect tissues from thermal damage due to abrasions from the proximal portions of the tip wherein the working part of the ultrasonic device, the end of the horn which contacts the tissue to be removed, is at the distal end of the tip. Such abrasions occur when the surgeon must direct the tip to a local area within a patient's body. As a consequence, lateral forces may be exerted on the horn particularly in liposuction or other minimally invasive applications where a long slender tip is needed. The result of such flexure is significant mechanical and thermal losses associated with less efficient transfer of mechanical energy to the tip of the device. In addition to efficiency losses, such lateral loading on the sheath and tip can cause rubbing and a scoring of the ultrasonic horn resulting in stress concentrations which can lead to cracking and shortened tip life.
The '903 patent addresses the problem of energy loss resulting from flexure by providing the plastic sleeve with inwardly projecting elements that are located so as to rest or contact the ultrasonic horn at a nodal point where longitudinal motion is at minimum. Alternatively, the '903 patent describes separate inserts which can be slipped over the horn and positioned at the vibratory nodes. The function of the supports is to maximize the transfer of energy to the tip of the horn and minimize undesirable energy transfer to the sleeve. The nodes are selected and are well known to be the points along the horn that produce the least vibration and are suited as a contact point between the sleeve and the horn.
In the aforementioned '903 patent, the sleeve is disposable and made of plastic, obviating the need for cleaning. However, the use of a plastic sleeve is not suitable for liposuction and minimally invasive surgery in that it tends to disintegrate from heating and vibration and thus may inject plastic particles into a patient where the particles can do harm. For example, burned plastic can be carcinogenic. Consequently, stainless steel is preferred. However, when a stainless steel sleeve is employed in the manner as taught by the '903 patent, the radially inwardly projecting elements, even though they are located at a nodal point, tend to score the horn. This scoring leads to stress points or lines and a subsequent cracking or breakage of the ultrasonic horn as previously described. It has been reported in the literature, that even with non sheath designs, ultrasonic tips have cracked and fallen into a patient during liposuction, resulting in at least a difficult retrieval situation during surgery. Even if the tip does not crack, the useful life of the tip is diminished because of the scoring at the nodes of the tip as a result of such contact. Such scoring can be accelerated by intentional contact with the support as taught by the '903 patent. If an annular ring is used that slips over the horn to contact the sleeve, significant mechanical and thermal losses can still occur due to the interface between the sleeve and horn. Moreover, the problem of fretting corrosion can occur at such an interface which can lead to further weakening of the integrity of the horn.
U.S. Pat. No. 4,6343,419 (Kreizman et al.) discloses an improved ultrasonic handpiece with an angled connecting body between the transducer and operative tip. The '419 patent discloses support means emanating radially from an inner wall of a support element to firmly hold the transducer in place. As with the '903 patent this arrangement does not solve the problem of scoring or weakening of the horn.
U.S. Pat. No. 3,956,826 (Perdreaux, Jr.) teaches the use of preferably a plastic outersleeve with a bore, annular groove, and a ring brazed onto the shaft of a horn at a nodal point. However, this approach does not solve the problem of fretting corrosion or efficiency loss from such an arrangement, and is designed so as to contact and support the sleeve from the horn.
U.S. Pat. No. 3,805,787 (Banko) describes an ultrasonic surgical instrument wherein a stainless steel sleeve surrounds the horn. The sleeve is suspended in cantilever fashion from a proximal end of the housing. However due to this type of support the problem of flexure still remains and scoring contact near the distal end of the sheath occurs.
A common disadvantage to the prior art is that the ultrasonic devices suitable for liposuction are not of a design that; a) reduces mechanical and thermal losses from flexure and dissipation, and b) adequately support a sleeve or sheath which protects the patient against undesirable thermal damage and abrasion of the horn while simultaneously preventing scoring and breakage of the tip along with capture should the tip break.
What is desired, therefore, is an ultrasonic device, particularly suitable for liposuction or minimally invasive procedures, which employs a design that will protect the patient from unwanted thermal damage and tissue abrasions due to lateral contact, reduce flexure, reduce efficiency losses, and prevent broken or disintegrated material associated with the device from entering the patient.